fix: Cleanup blog template images

[Greenheart]

Changes

• Remove old unused image from the blog starter template.
• Optimize image size for blog-placeholder-5.jpg (-94% size reduction)

(This contains no changeset as per instructions in https://github.com/withastro/astro/blob/main/CONTRIBUTING.md#making-a-pull-request)

Testing

• Verified that the blog stater template works as before by ensuring all five images display correctly at http://localhost:3000/blog when running the blog starter template
• The directory size should be slightly smaller than before.

Docs

Not applicable since this is just a content change for the blog template.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8279275

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[bluwy]

Looks great. Thanks!